This invention relates generally to electrical propulsion systems for diesel electric locomotives equipped with alternating current electric traction motors and, more particularly, to a method and apparatus for enabling continued operation of the locomotive in the event of a failure of a speed sensor.
In a conventional diesel electric locomotive, a thermal prime mover (typically a 16 cylinder turbo-charged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of alternating current (AC) traction motors whose rotors are drivingly coupled through speed reducing gearing to the respective axle wheel sets of the locomotive. The generator typically comprises a main three-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the diesel engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in three-phase armature windings on the stator of the alternator. These voltages are rectified to produce a controlled amplitude DC voltage and then applied to one or more PWM (pulse width modulation) inverters which control the effective frequency of alternating current to be supplied to the armature windings of the AC traction motors. The effective AC excitation frequency produced by the inverters controls the speed of the AC motors with power being controlled by pulse width modulation of the AC waveform.
In normal motoring; operation, the propulsion system of the diesel electric locomotive is so controlled as to establish a balanced steady state condition wherein the engine driven alternator produces, for each discrete position of a throttle handle, a substantially constant optimum amount of electrical power for the traction motors. In practice, suitable means are provided for overriding normal operations of the propulsion controls and reducing engine load in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capability of the engine at whatever engine speed the throttle is commanding. This response, generally referred to as deration, reduces traction power, thereby helping the locomotive recover from such temporary conditions and/or preventing serious damage to the engine.
In addition, the propulsion control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage ;and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits. Typically, both the output voltage of the rectifier connected to the output of the alternator and the output current from the rectifier are monitored with appropriate sensors which provide both a feedback control of the propulsion system operation and also serve to prevent over-voltage and/or over-current conditions. At low locomotive speeds, the traction motor armatures are rotating slowly so that their back EMF is low and requires a low voltage which can be supplied by the PWM inverters. On the other hand, the alternator voltage must be held at higher levels when the locomotive speed is high since the traction motors are rotating rapidly and require a higher voltage.
Each AC motor of a locomotive is coupled to drive a separate wheel-axle set and each motor is provided with controlled power from a corresponding one of a plurality of PWM inverters. The inverters are controlled from respective ones of a corresponding plurality of process controllers such as, for example, a microprocessor based control unit. While one control unit could be used to regulate operation of all inverters, multiple independent control units are desirable to provide redundancy, i.e., to allow the locomotive to operate under at least some amount of power in the case of failure of one or more control units. The control units rely on feedback signals from the motors for setting the appropriate power to each motor and to detect such undesirable events as a wheel slip/slide condition. Such feedback is commonly provided by motor or wheel speed sensors such as a tachometer coupled to the motor rotor or geared to the wheel-axle set.
In general, a preferred form of tachometer produces at least two signals which can be resolved in older to determine both wheel speed and direction of rotation. One such tachometer produces a pair of phase displaced signals, typically square wave signals, in which the frequency of each signal is representative of motor or wheel speed and the phase displacement between the signals provides an indication of the direction of rotation. With this form of tachometer, either signal can be resolved to provide speed information but both signals are needed to provide an indication of rotation direction. A problem arises whenever one of the signals fails, usually by a short-circuit, open circuit or an intermittent one of these events. If both signals are lost, the lack of feedback information requires shutting down of the AC drive associated with the failed tachometer. Since a common failure is loss of only one of the pair of signals, it is desirable to provide a method and apparatus to allow continued operation when one such signal is lost.